Use of blends of tertiary alkyl mercaptans in emulsion polymerization



Aprll 24, 1951 w. w. cRoucH ET A| USE 0F BLENDS OF' TERTIARY ALKYL MERCAPTANS IN EMULSION POLYMERIZATION 2 Sheets-Sheet l Filed Feb. 2, 1945 April 24, 1951 w. w. cRoucH ETAL 2,549,961

. usE oF BLENDs oF TERTIARY ALKYL MERCAPTANS IN EMULsIoN POLYMERIZATION Filed Feb. 2, 1945 2 Sheets-Sheet Patented Apr. 24, 1951 USE OF BLENDS OF TERTIARY ALKYL MERCAPTANS MERIZATION EMULSION POLY- Willie W. Crouch, Bartlesville, Okla., and Edwin G. Marhofer, Phillips, Tex., assignors to Phillips Petroleum Company, a corporation of Delaware Application February 2, 1945, Serial No. 575,819

1 Claim.

This invention relates to the production of high molecular Weight polymers. It is particularly applicable to the production of synthetic rubber by the polymerization of polymerizable organic compounds in an aqueous emulsion. In one of its more specific aspects this invention relates to the use of mixtures of certain mercaptans as modifying agents in the emulsion polymerization of butadiene-styrene and other related comonomer systems whereby the quality of the polymerizates is greatly improved.

Synthetic rubber is made by polymerization of polymerizable organic compounds under controlled polymerization conditions. The term synthetic rubber is used broadly to include the polymerizates of olefins, diolefins, styrene and its derivatives, alkyl esters of acrylic and alkacrylic acids (such as methyl methacrylate), and other compounds having at least one active CH2=C group. These compounds are polymerized alone or in admxture with one another to form products having some of the characteristic properties of -synthetic rubber. When a mixture of two or more of these compounds is subjected to polymerization conditions., a copolymer is formed in which the components form high molecular weight molecules by the linking together of the different individual component monomers. We have found that the synthetic product produced by the polymerization of Ia polymerizable organic compound is improved .bythe addition of a blend of tertiary aliphatic mercaptans to the monomeric compound to be polymerized prior to the polymerization thereof, This is particularly effective' in the polymerization of butadiene in an aqueous emulsion with suitable comonomers, for example, styrene, derivatives of styrene containing an active CH2=H group, acrylonitrile, methacrylonitrile, methyl acrylate, methyl methacrylate, etc., to form copolymers. Buna-S (or GR-S) is an example of the most important synthetic rubber so produced at the present time.

It is well known that copolymers of the Buna-S type are unsuited for use as synthetic rubber unless the emulsion polymerization is carried out in the presence of modifying agents. The general function of modifiersvis to eliminate or substantially reduce the formation between polymer units of cross-linkages leading .to the production of geltype products which render the polymerizates tough, hard and generally refractory toward subsequent process operations. Further beneficial effects of modifying agents are frequently manifested in increased polymerization rates. The most effective modifying agents heretofore known to the art have been selected alkyl mercaptans and especially the primary alkyl mercaptans having about 12 carbon atoms per molecule. y

It has now been found that tertiary alkyl mercaptans, especially those selected from the group having 8 to 16 carbon atoms per molecule, have a comparable beneficial modifying effect on Buna-Sl polymerizates. found that blends of two or more selected tertiaryI mercaptans from the above groupexert a modifying action superior in over-all effect to any of the individual mercaptans included in the blends. The polymerizates prepared in the presence of our novel blended modifier compositions are more uniform with respect to molecular weight range and therefore have properties which are superior to those of polymerizates modified in the conventional manner. Thus when a modifier of uniform carbon content is employed, the rate of depletion of modifier is such than an over-modified polymer of inadequate chain length is produced in the early stages of polymerization. Since excessive quantities of modifier have been consumed, cross linking sets in during the final phase of polymerization as is evidenced by a very rapid rise in average molecular Weight. Although both overand under-modification have resulted, the average molecular weight of the final product is sufficiently high to permit processing operations; however, such polymers have poor aging properties. This lack of lcontrol over the rate of reaction of modifier results in a product containing a proportion of polymer of objectionably low molecular weight and another portion having excessively high values. With the blended modiers of the present invention, the above objections are overcome due to the fact that the mercaptan components are consumed at different rates. The net effect of our blended modifiers is to produce a relatively uniform modification with elimination of cross linking duringthe later stages of polymerization. Thus, finished polymer of the Same average molecular weight is much more homogeneous when modified with a blend Aof mercaptans of different molecular weights than would be the case if any one component of the blend were used alone. In this manner, we are able to control the average molecular weight of the polymer at all stages of conversion and thereby effect a substantial improvement in the quality of the product.

An object of this invention Ais to provide a novel process for the production of high molecular weight polymers.

Another object is to provide an improved process for polymerization of polymerizable organic compounds in an aqueous emulsion. f

Still another object is to .provide such .a process which is particularly useful for the production of polymerizates of the Buna-S type.

A further object is to provide such a process .in `which a blend of `two or more tertiary aliphatic mercaptans used as a modifier for the` poly- I merization.

Inone embodiment of the present invention, a Conventional polymerization recipe is employed.

Unexpectedly, however, We have'`- 3 4 l While the preceding representativel mercaptan Component Patsbl'f Wtfractions exert a modifying action comparable Y With other commercial modifiers, we have now ggjne gg found that polymers of superior characteristics snep I .11 8 can be realized by utilizing blends of our tertiary {,gaesrsmm Persulfate 18g-3 mercaptans' to give a modifier of variable carborr Mermlmn "II variable content. The modifying effect of these blends is such as to result in polymer characteristics en- Temperature 50 C- Y v i tirely different from the effect that could be pre- The quantity of modifier used in any recipe is I dicted Vfrom the individual behavior of the blend"- dependent on the type of mercaptan or mercapf components. The reason for this synergism in. tans used and is determined by experiment; For modifying action is not known, but it is apparentexample, to obtain an lacceptable polymerizate for ly related in some obscure fashion to relative reuse in synthetic rubber with a Mooney viscosity action rates. By virtue of this newly discovered-V Qf 45v55 the conversion is stopped at about 77 per 15 method of proper blending of tertiary mercapf cent of the monomer charge. These conditions tans, we are n 'ow yable to obtain a smooth rate of areselected to give a'gel-free polymeric productmodification throughout the entire polymeriza-r having an adequately high average molecular Y tion period. Under the conditions' used hereto-t weight. The quantity of modifier is, therefore, fore, where a non-uniform rate of modifier de determined by these criteria. Excessive quantipletion occurs, the polymer product represents d ties of modifier require higher conversions in combination of over-modified and under-modi order to realize the` specified Mooney viscosity; fied components. It is obvious that a more honioe however, when this is done a polymer may be geIIeOUS polymer, of the Same average molecular` obtained which isnot readily amenable tothe Weight as obtained With previous unitary modi# rubber compounding and vulcanization opera- 2 fiers, could be produced by simply increasing thev tions. It can be seen, therefore, that the quanpolymer SZe in the early Stages of D0lYme1ZdtQI1`- tity of any givenmercaptan modier must be and eliminating to a large extent the extremely carefully controlled within somewhat narrow high molecular Weight material ordinarili7 DI" limits. duced in the latter stages of polymerization. In*- Appiieation of individual tertiary mei-Captaris this connection it has been Shown by Kemp and as modifiers in the standard GR-S recipe is suit- Stratff (Ind- Eng- Chem- 36, 707 (1944 that? ably accomplished in the following generalized the 111051? desirable polymers of the Buna-S type' procedure. kAn emulsion of the recipe ingredients ere those which do not contain large amounts of" along with the necessary quantity 0f modifier is material of either very high or very low molecular agitated for 12 hours at 50 C. The resultant 35 Weight. That our blended modifiers tend to prof latex is treated with phenyl-beta-napiithyiamine duce a more uniform product is attested by dater antioxidant followed by coagulation. The crude to be presented later wherein the ratio of polymer' polymer lis washed and dried in preparation for molecular Weights at Various stages of conversioir evaluation and/or subsequent process Stepg In to that of the final product are compared with this manner,v gel-free products meeting the cri- 40 similar data derived from experiments using coner teria of monomer conversion, Mooney viscosity VelltOIlal unitary modifiers. and processabiiity are obtained with the Yfoliowing In order to demonstrate the improved results tertiary aliphatic mercaptans: attributable to Vour blends of specified tertiary A mercaptans, comparable polymerization experi- Modef Partsbywt. 45 ments were carried out using as modiers the Y fractionated samples of Ca, C12, C14 and C16 ter- {igyelcefeasg: tiary mercaptans previously described. VThe ret-tetradecyimereeptens 0,34 Sults obtained with the individual unit modifiers werevthen compared with experiments wherein "I 'he above identified mercaptans are derived from 50 blends of the mercaptan fractions were used for mixtures of isomeric olens by direct catalytic the same purpose. In each test run, the standard condensation with hydrogen sulfide.- The so- GR-S recipe was employed along with suicient called t-octyl mercaptans, for example,A are added modifier to result in about 77 per cent concomprised of many different isomers having 8 version of monomers to polymer, having a Mooney carbon atoms per molecule. We have found that viscosity of Ll5--55. In experiments involving nonsuch fractions of narrow boiling range tend to beblended modifier, the proportions of mercaptan have as a single mercaptan when used as a polyare those previously given. The difference inbemerization modifier. The same condition is true havior of the various modifiers during the progress Qf tl'le t-.dodeeyl and t-tetradecyl mercaptans of the polymerization was followed by periodic listed above. In order to specifically define the withdrawal of samples to determine extent 'of synthetic tertiary aliphatic mercaptans of this inconversion and intrinsic viscosity of the product Verwen, phySlCal properties of selected molecular polymer. Since intrinsic viscosity is .a measure felght groups are given in the subjoined tabulaof molecular weight (of. Kemp and Peters, Ind. *510m Y Y, Y f Eng. chem., `23. 1263 1941, the experimentar t-Mercaptans.-r.r. C8 0n C14 .Cia l itsirazriitiz--- iii ist 0- RsH sulfur, wtper oent 21. 6 1519 11. 9 1o s RSE pur ty, wt. per cent 97+ 96.8 85, f i502 Dis'fllttign o F- l(760 (5 mm.) 2(5 mm.) 2(5 mm.)

ggg ng- 319 A :et is si *9,5%710111 333 212 252 deo.

l' .ASTM DSG-40.

2 Rubber Reserve Company Test Method L, M. 2.1M.A

results may be reported in terms of average molecular Weights of polymer through the use of a simple mathematical relationship.

TABLE I Variation in molecular weights of GR-S polymers,

modified with various tertiary mercaptan com- 1 The blend composition consisted of l part Cs, 2 parts C12 and 2 Darts C14 tert. mercaptans.

In the critical polymerization range lying between 50 and 77 per cent conversion of monomers, the rapid transition from relatively low to relatively high molecular weight polymers has been greatly reduced by the use of blended modifiers without reducing the ultimate average molecular weight. The results achieved through the use of our blended modifiers are even more striking when the above data are presented in such a form as to show the actual data obtained with blended mercaptans in comparison with a calculated expected average effect. Thus, knowing the proportion of each mercaptan in the three component blend, the effect of each component on the molecular Weight of GR-S polymers and assuming that each component exerts its own modifying action without synergistic action, a hypothetical set of data similar to that of Table I can be calculated. Such calculated data are pre sented in Table II along with data obtained using the actual blend. Inspection of the table reveals the unexpected and beneficial synergistic action of the blended tertiary mercaptans.

TABLE II Comparison of predicted and actual modifying action of blended t-mercaptans Average Molecular Weight of Polymers Monomer conversion 50% 60% 70% 77% lEIypothcticalMercaptanblend1 123, 000 161, 000 246, 000 343, 000 Actual Mercaptan blend 1 152, 000 224, 000 328, 000 352, 000

1 l part C5, 2 parts C12, 2 parts C14 tert. mercaptans.

TABLE III Homogeneity of modified polymer product ea:- pressed as percentage of final molecular weight Monomer Conversion ..1 50% l 60% l 70% 77% Modiers:

Cs tert. mercaptans. 52. 8 95.0 100 C12 tert. mercaptans. 40. 6 69. 2 100 CH tert. mercaptans. 50. 2 62. 5 100 Cs, C12, C14 mercaptan blend l.. 63. 8 95. 6 100 1 l part G5, 2 parts C12, 2 parts C14 tert. mercaptans.

TABLE IV Homogeueity of modified GR-S' polymers. Calculated versus experimental values Percentage of Final Molecular Weight Monomer Conversion 50% 60% 70% 77% Hypothetical mercaptan blend 35. 9 47, 0 7l. 8 100 Experimental mercaptan blend 43. 3 63.8 95.6 100 By way of further illustration of the unexpected. results achieved with the ternary mixture of tertiary mercaptans, Figures l and 2 show the superiority of the aforesaid mixture over its individual components.

The preferred modifying agents of this process comprise the tertiary aliphatic mercaptans containing from 8 to,16 or more carbon atoms per molecule. While pure individual tertiary mercaptans are not excluded, such compounds are ordinarily not available. The most abundant and practical source of tertiary mercaptans is from the catalytic addition of hydrogen sulde to olefinic polymers such as may be derived from catalytic polymerization of refinery C3, C4 and C5 olens. These olefinic polymers are fractionated into narrow boiling-range cuts corresponding in average molecular weight to octenes, nonenes, decenes, undecenes, dodecenes, etc., prior to conversion to the respective mercaptans. The complexity of isomeric types of the same carbon content in any one fraction virtually precludes commercial isolation of any one pure mercaptan isomer. However, it is known that mercaptan isomers so produced are tertiary in configuration to an extent greater than per cent. While any one of the mercaptan groups such as Ca, C12, C14, Cie, etc., may be used alone as modifiers, the unusual advantages described herein are realized only when blends of two o1' more groups of isomers are employed. The direct manufacture of our mercaptan blends from olens of wide boiling range is precluded because of operational and purification difficulties.

While considerable attention has been given to the modifying effect of ternary blends of tertiary mercaptans it is not to be implied that the were as follows: v

EXAMPLE I The relative modifying activity of individual tertiary C12 and C14 mercaptans and a blend of these materials was determined under compare able reaction conditions using'equivalent quantities of mercaptan in each instance. The test procedure was as follows: a series of emulsions comprising 75 parts butadiene, 25 parts styrene, parts soap alies, 0.3 part potassium persulfate and 180 parts Ywater was prepared; Ymodifier was added and polymerization effected with constant agitation at 50 C. for variable lengths of time in order to follow the progress of modification withr monomer-conversion; the polymer was recovered and its intrinsic viscosity was determined. In this manner the effectiveness of tertiary C12 and C14 mercaptans was compared with a 68-32 blend of the respective mercaptans, i. e., 68 parts by weight of the C12 mercaptan admixed with :32 parts by weight of the C14 mercaptan. The quantities of modifier employed in these runs Modifier vParts by wt.

C12 t-mercaptan 0A 28 C14 t-mercaptan.. 0. 40 C12-C14 blend 0. 31

The general results of these tests and the-superiority of the blended modifier is illustrated in Figure 3 Where polymer intrinsic viscosities are Y and wearing properties.

EXAMPLE II Employing Ythe technique of Example I and the same basic recipe, the modifying action of individual tertiary C12 and C16 mercaptans was com- 8 paredfwith a` 56-44.v blend, respectively, ofthese mercaptans. The following amounts of modifier were used in the three series of tests:

Modifier Parts by wt.

C12 tert. mcrcaptans 0.28 C16 tert. mercaptans 0.45 C12-C15 blend 0. 36

Figure 4 represents a graphicalpresentation of data relative to polymer modified with the blend and with the C12mercaptans alone. It can be seen that the polymer modified with the blend has higher intrinsic viscosity and therefore higher molecular weight at low conversion than polymer modified with C12 mercaptansA alone. As in the previous example, a more uniform and more desirable product was prepared. A curve for polymer modied Vwith C16 mercaptans could vnot be constructed since none of .the polymer was sufliciently modified to be completely soluble in benzene. Thus, in spite of the fact that a relatively large amount of C16 mercaptan modifier Was employed, ,gel formation was pronounced throughout the polymerization period. However, when this C16 tertiary mercaptan is admixed with C12 tertiary mercaptan, a highly satisfactory land superior modifying agent results.

We claim:

In the'manufacture of synthetic'rubber by th polymerization of a monomeric material comprising a major amountl of butadiene and a minor amount of styrene in aqueous emulsion in which a modier is employed to produce a gel-free polymerio product, the improvement which comprises polymerizing said monomeric material to between 50 and 77 per cent conversion and utilizing as the modifier a blend of tertiary alkylmercaptans, said blend containing 56v per cent by weight of tertiary C12 mercaptans, and 44 per cent by Weight of tertiary C16 mercaptans, in an amount such that the Mooney viscosity of the resulting polymeris 45 to 55. Y I

WILLIE W. CROUCl-I. EDWIN G.I MARHOFERL REFERENCES CITED The following references are of record in the file of this, patent:

UNITED STATESPATENTS Number Name Date 2,209,169 Mikeska July 23, 1940 2,378,030 Olin June 12, 1945 2,384,969 Serniuk Sept. 18, 1945 2,396,997 `Fryling Mar. 19, 1946 2,401,346 I Fryling June 4, 1946 2,416,440 Frylng Feb. 25, 1947 

